As an emerging candidate for next-generation communication systems, reconfigurable intelligent surfaces (RISs), also called intelligent reflecting surfaces (IRSs), have attracted significant interest from both academia and industry[1]}. By intelligently tuning the phase shifts of the impinging wave, an RIS can constructively strengthen the desired signal power or deconstructively weaken the interference signal, which yields an appealing passive beamforming gain[2]}, [3]}, [4]}.
